This invention relates to air intake measuring devices for internal combustion engines, and more particularly it is concerned with a device for measuring an air flow rate comprising a resistor the characteristics of which depend on temperature which resistor is mounted in a suction conduit of an internal combustion engine for measuring the flow rate of air by use of the variations in amount of heat carried away by the air intake from the resistor as the resistor is caused to generate heat.
One example of devices for measuring an air intake for internal combustion engine is disclosed in U.S. Pat. No. 3,824,966 which uses a bare platinum wire as an air flow rate sensor element mounted in a main suction conduit. It is usual practice to use a fine platinum wire of about 0.05 mm in diameter to increase the accuracy with which the flow rate of air is measured. The finer the platinum wire, the higher becomes the accuracy with which the air intake can be measured. However, when the platinum wire used is too fine, the platinum wire is succeptible to influences exerted by backfires from the internal combustion engine, so that the platinum wire might be elongated and changes might be caused to occur in the resistance value. In worst cases, a break-out of the platinum wire might result.
Air intake of an internal combustion engine has pulsations, so that the output signal of a device for measuring the flow rate of air shows corresponding changes. It is not necessary for an air intake measuring device for an internal combustion engine to obtain an instant value of the flow rate of air, and it is rather preferred that the amount of air drawn into the engine in one suction stroke be accurately measured.
To this end, it has hitherto been customary to integrate a signal wave form produced by a measuring device over a predetermined period of time. However, this practice has suffered such disadvantage that the signal processing circuit becomes complex, making it difficult to obtain results at high speed.